Two cyclometalated Pt(ii) complexes showing reversible phosphorescence switching due to grinding-induced destruction and crystallization-induced formation of supramolecular dimer structure

Complexes [Pt(dfppy)(pbdtmi)]PF6 (1) and [Pt(ppy)(pbdtmi)]PF6 (2) have been constructed based on dithienylethene-based N^N ligand pbdtmi, showing supramolecular dimer structure in which two coordination cations connect each other through π⋯π stacking interaction. The crystalline state samples of both 1 and 2 reveal strong phosphorescence (emission peak: around 579 nm for 1, and 551 nm for 2). Interestingly, a grinding treatment for either 1 or 2 leads to phosphorescence switching from on-state to off-state. The subsequent crystallization with toluene recovers the initial on-state. This work discusses the relationship between the supramolecular dimer structures and the related phosphorescence switching behaviors in 1 and 2, and also explores the photochromism of pbdtmi, 1 and 2.


Introduction
Cyclometalated Pt(II) complexes have attracted considerable attention due to their luminescence with potential applications in luminescence switching materials, 1,2 sensing, 3,4 bioimaging, 5,6 and organic light-emitting diodes (OLEDs). 7,8 In order to modulate the structures and the related luminescence of Pt(II) complexes, one effective strategy is to incorporate various cyclometalated ligands (e.g. 2-phenyl-pyridine and its derivatives) and/or ancillary ligands L^X. 9,10 The other is to control intermolecular interactions, such as Pt/Pt interaction, 11,12 hydrogen bond, 13 and p/p stacking interaction, 14,15 which can signicantly affect the luminescence of Pt(II) complexes.
It is well known that dithienylethene (DTE) is a class of fascinating organic compounds, showing photochromism due to light-irradiation-induced structural transition between open form and closed form (Scheme S1 †). 16 So far, some DTE-based Pt(II) complexes have been reported, in which various DTE ligands coordinate with Pt(II) ions through some functional groups such as alkynyl group, 17-21 pyridyl group, 22 1,10-phenanthroline unit, 23 2-(thiophen-2-yl)-pyridine moiety, 24 2-(2-pyridyl)imidazole moiety, 25 and 1,3-di(2-pyridyl)benzene unit. 26 Some complexes were found to exhibit phtochromism, and even signicant switching of either luminescence or nonlinear optical (NLO) property due to the open/closed isomerization of DTE unit. 18,22,23,25,26 However, no document has so far reported a DTE-based Pt(II) complex showing supramolecular structure and the related functional property.

Syntheses and structural characterizations
Complexes 1 and 2 were readily synthesized through the reaction of pbdtmi, [Pt(dfppy)(SEt 2 )Cl]/[Pt(ppy)(SEt 2 )Cl] and AgPF 6 in CH 2 Cl 2 at 40 C for one day. The two complexes can also be prepared through the reaction of pbdtmi and the corresponding dichloride bridged dimer [Pt(dfppy)Cl] 2 /[Pt(ppy)Cl] 2 , and the following anion exchange of Cl À with PF 6 À . However, this synthesis approach needs to use tedious silica column chromatography to purify the crude product. The structures of 1 and 2 were characterized by 1 H NMR, IR spectra, elemental analyses, and crystal structures.

Crystal structures of 1 and 2
The single crystals of both 1 and 2$C 7 H 8 (C 7 H 8 ¼ toluene) were grown in a CH 2 Cl 2 -toluene mixture, and measured by X-ray crystallography. The crystallographic data, the selected bond lengths and bond angles are summarized in Tables S1-S3 (ESI †). Complex 1 and 2$C 7 H 8 crystallize in the monoclinic space group P2 1 /n and P2 1 /c, respectively, showing similar molecular structures ( Fig. 1 and S5 †). A neutral N^N ligand pbdtmi coordinates to a {Pt(dfppy)} + unit in complex 1, and a {Pt(ppy)} + unit in complex 2. The resultant [Pt(dfppy/ ppy)(pbdtmi)] + cation and its counter ion PF 6 À are held together by electrostatic interaction. Each Pt(II) ion in 1 and 2 shows a distorted square planar geometry. Two of the four coordination sites are occupied by the C1 and N1 atoms from a dfppy À / ppy À ligand. The remaining two coordination sites are occupied by pyridine nitrogen N2 and imidazole nitrogen N3 from a pbdtmi ligand. The dihedral angle between N1-Pt1-C1 plane and N2-Pt1-N3 plane is 3.5(1) in complex 1, and 2.9(1) in complex 2. The coordination of dfppy À /ppy À and pbdtmi towards the Pt(II) ion results in the formation of two vemembered rings ( Fig. 1  . This can be due to the fact that the N3 atom is trans to the sbound C1 atom ( Fig. 1 and S5 †). 27 Complexes 1 and 2 exhibit similar stacking structures ( Fig. 2 and S6 †). Two neighboring [Pt(dfppy/ppy)(pbdtmi)] + cations in these complexes connect each other through the p/p stacking interaction between two pyridine rings from dfppy À /ppy À ligand and pbdtmi ligand [centroid-centroid distance: 3.544(1) A in 1, and 3.595(1) A in 2], thus forming supramolecular dimer structures ( Fig. 2 and S6 †). The Pt/Pt distance is 3.627(1) A in 1, and 3.698(1) A in 2, hence there is no Pt/Pt interaction. 28,29 These dimers stack through van der Waals interactions, and the inter-dimer space is lled with PF 6 À anions and/or toluene molecules ( Fig. S7 and S9 †).

Electronic absorption spectra
Complexes 1 and 2 in CH 2 Cl 2 have similar absorption spectra ( Fig. S9 and Table S4 †), showing two strong bands (around 306 and 359 nm for 1, and around 310 and 363 nm for 2) and a weak absorption tail (toward 448 nm for 1, and toward 456 nm for 2).
In contrast, compound pbdtmi only exhibits a broad band at 320 nm. For complexes 1 and 2, the absorption bands at 306 nm and 310 nm can be assigned to ligand-centered ( 1 LC) transitions (dfppy À /ppy and pbdtmi ligands), while the bands around 359 nm and 363 nm should be a combination of metal-to-ligand charge transfer ( 1 MLCT) and ligand-centered ( 1 LC) transitions, because of the high extinction coefficient (3 ¼ 9.1 Â 10 3 M À1 cm À1 for 1 and 3 ¼ 1.1 Â 10 4 M À1 cm À1 for 2). 30 The weak absorption tails in both 1 and 2 are mainly attributed to 3 Table S5 †). It should be noted that the emission bands of both 1 and 2 are very broad (Fig. 3), indicating typical 3 MLCT character. The phosphorescence emissions from the crystalline samples of 1 and 2 can be assigned to the inter-molecular p/p stacking interactions ( Fig. 2 and S6 †) in the resultant supramolecular dimers. This can be further conrmed by the following grinding and crystallization experiments. It is well known that intraligand (IL) charge transfer can result in luminescence quenching in some metal complexes. 33 The crystalline samples of 1 and 2 are luminescent, mainly due to the fact that their dimer structures invalidate the intraligand (IL) charge transfer from pyridine-imidazole moiety to thiophene unit in ligand pbdtmi. 34 These dimer structures can be destructed by grinding treatment, thus the intraligand (IL) charge transfer can uently occur in the grinding samples 1g and 2g, and leading to luminescence quenching (Fig. 4). Considering that toluene can promote the crystallization of complexes 1 and 2 (see synthesis details), we add a drop of toluene into a grinding sample (1g or 2g). The resultant crystallization samples 1c and 2c show yellow emissions, indicating the recovery of the initial supramolecular dimer structures. Clearly, the phosphorescence of 1 and 2 can be reversibly switched between on-state and off-state by using grinding and crystallization processes to modulate the supramolecular dimer in these complexes, which is clearly different from the widely reported grinding-induced luminescence switching between two kinds of emission colors in some reported Pt(II) complexes, for example, a series of pinene-containing Pt(N^N^C)Cl complexes. 35 To our knowledge, complexes 1 and 2 are two exclusive dithienylethene-based Pt(II) complexes showing reversible grinding-and crystallization-induced phosphorescence switching.
Moreover, the crystalline sample of complex 1 reveals longer emission length than that of complex 2 (emission wavelength: 579 nm for 1, and 551 nm for 2), although there are two drawelectron F substituent groups in the molecular structure of 1. This can be due to relatively strong p/p stacking interaction in complex 1 [see Fig. 2 and S6, † centroid-centroid distance between dfppy À /ppy À ligand and pbdtmi ligand: 3.544(1) A in 1, and 3.595(1) A in 2].

Photochromism study
Ligand pbdtmi shows photochromic behavior in solution at room temperature, which is conrmed by its 1 H NMR spectra and UV-vis absorption spectra (Fig. 5, S1, S2, S12 and S13 †). Before and aer irradiation with 325 nm light, the 1 H NMR spectra of pbdtmi in CDCl 3 show clear changes in peaks of aromatic H atoms and methyl groups belonging to two thiophene groups (Fig. 5). The irradiation resulted in downeld shi for the chemical shis of the corresponding aromatic protons, while upeld shi for the chemical shis of the methyl groups at two thiophene groups. Moreover, upon irradiation, the CDCl 3 solution of pbdtmi changed its color from colorless to brown (Fig. 5). These changes indicate that ligand pbdtmi has photochromic property, which is further conrmed by the absorption-spectrum change of pbdtmi in CH 2 Cl 2 upon irradiation with 325 nm light. Under the irradiation with 325 nm light, the original strong absorption band at $320 nm shows slight decrease in intensity, and forms a new broad absorption band around 595 nm (Fig. S12 †). This indicates the structural transition of pbdtmi from open form to closed form. This new absorption band could slowly disappear under the condition of being irradiated with 600 nm light or staying in the dark at room temperature (Fig. S13 †), indicating that the structural transition of pbdtmi from closed form to open form.
Compared to free ligand pbdtmim, neither 1 nor 2 reveals obvious change in UV-vis absorption spectrum upon irradiation with 365 nm light ( Fig. S10 and S11 †), indicating that the two complexes have no photochromic property. The absence of photochromism in both 1 and 2 could be mainly due to the following two factors. 31 The one is that the UV excitation energy is transferred to {Pt(dfppy/ppy)} + unit, resulting in the complete absence of photocyclization process of the ligand pbdtmi. The other one is that the coordination of pbdtmi to {Pt(dfppy/ppy)} + unit leads to a crowded space which hinders the ligand pbdtmi from taking part in the photocyclization reaction.

Conclusions
In summary, two dithienylethene-based Pt(II) complexes [Pt(dfppy)(pbdtmi)]PF 6 (1) and [Pt(ppy)(pbdtmi)]PF 6 (2) have been designed and synthesized. Their crystal structures indicate that two neighboring [Pt(dfppy/ppy)(pbdtmi)] + cations connect each other through p/p stacking interaction, thus forming supramolecular dimer structure (Fig. 2 and S7 †). Both 1 and 2 are nonluminescent in CH 2 Cl 2 , but their crystalline state samples reveal strong phosphorescence (emission peak: around 579 nm for 1, and 551 nm for 2), mainly due to their supramolecular dimer structures. Interestingly, these crystalline samples show reversible phosphorescence switching between on-state and off-state upon grinding and crystallization with toluene, because of the grinding-induced destruction and crystallization-induced formation of supramolecular dimer structure. Our work indicates that the construction of supramolecular dimer structure is one of feasible approaches to design and synthesize smart luminescent coordination complexes, which can respond to external stimuli (e.g. grinding, crystallization).

Materials and methods
Compounds [Pt(dfppy)(SEt 2 )Cl] and [Pt(ppy)(SEt 2 )Cl] were prepared according to the literatures. 36 All other reagents were commercially available and used without further purication. Elemental analyses were performed on a PerkinElmer 240C elemental analyzer. IR spectra were obtained as KBr disks on a VECTOR 22 spectrometer. The 1 H NMR spectra were recorded at room temperature with a 400 MHz BRUKER spectrometer. UV-vis absorption spectra were measured on a Cary 100 spectrophotometer. Luminescence spectra were measured using a Hitachi F-4600 uorescence spectrometer. The luminescence lifetimes were measured at room temperature on a HORIBA FL-3 Spectrouorometer with a 370 nm LED pulsed from a NanoLED resource. The luminescence quantum yield of pbdtmi in CH 2 Cl 2 solution was measured by a relative method by comparison with a standard, a solution of quinine sulfate in 0.5 M H 2 SO 4 (F ¼ 54.6%, l ex ¼ 366 nm). 37 The quantum yields of the crystalline samples of 1 and 2 were measured at room temperature on a Horiba FL-3 spectrouorometer.

X-ray crystallographic study
Single crystal of dimension 0.12 Â 0.10 Â 0.10 mm 3 for 1, and 0.21 Â 0.16 Â 0.13 mm 3 for 2$C 7 H 8 (C 7 H 8 ¼ toluene) were used for structural determination on a Bruker SMART APEX CCD diffractometer (l MoKa ¼ 0.71073 A) for 1 at room temperature, and on a Bruker D8 Venture diffractometer (l GaKa ¼ 1.34138 A) for 2$C 7 H 8 at 193(2) K. A hemisphere of data were collected in the q range 2.274 to 27.488 for 1, and 2.529 to 53.785 for 2$C 7 H 8 using a narrow-frame method with scan width u ¼ 0.50 for 1 and u ¼ 0.90 for 2$C 7 H 8 , and an exposure time of 10 s per frame. Numbers of observed and unique reections are 14 076 and 7311 (R int ¼ 0.0465) for 1, and 49 312 and 6609 (R int ¼ 0.0668) for 2$C 7 H 8 , respectively. The data were integrated using the Siemens SAINT program, 38 with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Multi-scan absorption corrections were applied. The structures were solved by direct methods and rened on F 2 by full matrix least squares using SHELXTL. 39 All the non-hydrogen atoms were located from the Fourier maps, and were rened anisotropically. All H atoms were rened isotropically, with the isotropic vibration parameters related to the non-H atom to which they are bonded. In the structural renements of 1 and 2$C 7 H 8 , ISOR, SIMU and SADI were used to rene disordered lattice toluene molecule, PF 6 À ion and thiophene group. The crystallographic data for compounds 1 and 2$C 7 H 8 are listed in Table S1, † and selected bond lengths and bond angles are given in Tables S2 and S3. † CCDC 2122586 and 2099083 contain the supplementary crystallographic data of 1 and 2$C 7 H 8 , respectively.

Conflicts of interest
There are no conicts declare.